The existing fleet of general aviation spark ignition piston engines, as well as new engines currently being delivered, and engines which are overhauled for use as replacements on existing aircraft, typically operate with a stoichiometric rich air/fuel mixture ratio, often abbreviated as the “AFR”. Quite simply, this means that with respect to the amount of fuel, there is not enough air in the rich AFR mixtures to completely react with all of the fuel molecules present in the air/fuel mixture being fed to the engine. That means that the fuel provided to the engine is not completely utilized in the chemical reaction of fuel burn, and consequently, it is clear that such a condition does not optimize the amount of work provided for the amount of fuel consumed, as measured by the brake specific fuel consumption, often abbreviated as “BSFC”.
Importantly, existing general aviation aircraft engines are typically designed and configured so that when operated at full power, the aforementioned AFR is set to a relatively rich condition. Further, such engines are typically designed and configured so that when operated at cruise power, the AFR continues to be maintained relatively fuel rich, albeit often slightly less rich than is the case at a “full power” mixture setting.
Since maximum engine output (e.g. horsepower available for takeoff) has been the most significant limiting constraint for general aviation aircraft in terms of maximum takeoff weight and climb performance, especially for piston powered aircraft and rotorcraft, aircraft piston engines have typically been designed and operated to provide their maximum horsepower output (i.e., maximum BHP) under takeoff conditions. For this reason, an historically consistent configuration utilized for such engines has been that the engines have been designed and operated so that the AFR has been set rich, and often quite rich, as heretofore believed necessary to maximize available horsepower at takeoff, as well as to maximize available horsepower during the climb portion of a typical flight profile. An additional (and necessary reason under prior art practices) for operating these engines during takeoff and climb at rich AFR is that such rich AFR have been required in order to provide adequate control of cylinder head temperatures, in order to prevent overheating and to provide adequate margins from detonation. In many cases, instructions as to the required rich AFR for such engines has been set forth in the engine manufacturer's operational manuals, and as further and more definitively provided by the manufacturer of aircraft in which such engines are utilized.
Attention is directed to FIG. 1, where those skilled in the art of aircraft and rotorcraft piston engine design and operation will recognize the classic relationships between equivalence ratio (defined below), which is representative of the air fuel ratio (“AFR”) used in an engine, the brake horsepower (“BHP”), brake specific fuel consumption (“BSFC” and also defined below), and cylinder head temperatures (“CHT”), for a typical spark ignition aircraft piston engine. Such curves might vary somewhat, depending on the qualities of the fuel being combusted, the spark timing advance, and the actual mass of airflow through the cylinder. However, in so far as I am aware, general aviation spark ignition piston engines currently are configured to operate at full power during the critical takeoff and initial climb phase of flight with the AFR operated at or near a full rich condition, with operating parameters as indicated in FIG. 1 at points 1A (showing BHP), 1B (showing CHT), and 1C (showing BSFC). In many instances, various makes and models of such engines are actually operated at still richer mixtures than those indicated in FIG. 1 in order to avoid detonation and to provide for adequate cooling of the materials of construction of the engine, particularly the piston, cylinder, and exhaust valve during critical phases of flight.
It can be observed from FIG. 1 that another desirable setting for the AFR might be at the location indicated by points 1D (showing BHP), 1E (showing CHT), and 1F (showing BSFC). However, if the AFR were adjusted to operate in an area generally described by the points just mentioned, while the engine would operate much more efficiently (i.e., better BSFC), and slightly cooler (lower CHT shown at point 1E than at point 1B), and with reduced CO2 emissions, the available engine horsepower (BHP) would decline by some 8 to 10 percent, which for example can be seen by comparing the BHP at point 1A with the BHP at point 1D. Such prior art aircraft engine lean conditions may be (and have been) tolerated or, in a limited number of cases, encouraged, during the cruise portion of a flight. However, such a loss of available horsepower, if that were the situation during the critical takeoff and climb phases of flight, is generally considered to be unacceptable. That is because, in terms of the performance of the aircraft, such decreased horsepower negatively affects the takeoff distance quite significantly, and also markedly increases the distance required for the aircraft to climb and to clear obstacles in the takeoff path. Unfortunately, even if such a performance reduction as just described was accepted by the aircraft operator, in order to legally operate such aircraft with the same engines but set up to run at such a reduced maximum available takeoff BHP, many existing aircraft would have to be recertified. That process would be quite expensive, and would entail going through an extensive regulatory recertification process to obtain a “supplemental type certificate” for the use of such a reconfigured engine in the aircraft. In some case, re-certification at a lower horsepower level would be practically impossible due to performance constraints. In the United States, and most countries, the certification activity is an expensive governmental process. In the United States, it is administered by the Federal Aviation Administration (the “FAA”).
Further, if spark ignited piston engines reconfigured as just described above were utilized in twin engine general aviation aircraft, a BHP loss in the range of 8 to 10 percent, when compared to the original “as certified” engine available takeoff horsepower (e.g. see point 1A as compared to point 1D in FIG. 1), would almost always be unacceptable. That is because even such a relatively modest amount of reduction in horsepower would, in many cases, virtually eliminate the ability of the aircraft to continue to climb with an acceptable climb rate while using only a single remaining good engine, in the event it becomes necessary to shut down one of the two engines due to a mechanical emergency.
Consequently, there still remains an as yet unmet need for an aircraft engine design, and a method for operation of such engines, that takes full advantage of the mechanical design components with respect to mass flow of air into the engine, and materials of construction utilized, that is capable of operating at lean AFR conditions, with good compression ratios, in a stable and highly efficient manner in all flight operating conditions. In order to meet such need and to provide a method for the design and operation of engines that can reliably achieve such operational conditions, it has become necessary to address the basic technical challenges presented in order to develop workable operating conditions, and methods for maintaining such conditions in spark ignition aircraft piston engines. Thus, it would be advantageous to provide for aircraft engines that can achieve the same BHP output during takeoff as counterpart (e.g. same or virtually identical engine specification) prior art rich AFR operating engines, but which can be operated at reduced fuel burn, and with less wear and tear on the mechanical components of the engines, as well as routine operation with reduced carbon foot print. Moreover, it would be advantageous to accomplish such goals while providing an engine suitable for drop-in substitution, or while providing a procedure for modification or rebuild of existing engines, which provides such advantages, in order to minimize the extent, complexity, and cost of any required recertification efforts of the critical high power performance portion of the operating envelope of existing aircraft.